The Shorrocks Supercharger

THE Shorrocks is perhaps the most commonly seen supercharger on small drag and sprint bikes. When I first became interested in the sport as a teenager, one could be mistaken for thinking it was the only supercharger.

The unit seems to have been developed by Chris Shorrocks, initially in the 1930's, at a time when supercharging was much to the fore. The sectioned drawing shows a typical unit of the most popular 'horseshoe' 360° type. That is, the inlet and outlet ports are side by side and the charge flows through the casing in a horseshoe path. One of the ports can be seen at the top with four mounting studs. There was a straight through i.e. 180° model and, rarest of all a, 90°version. The latter was developed to fit the limited space in a VW Beetle engine compartment. Three sizes were available;

• C75 - 750cc swept volume.
• C142 - 1420cc swept volume
• C250 - 2500cc swept volume.

Behind the double pulley in the end casing, note the roller bearing. A ball bearing supports the other end of the inner drum. The drive shaft is very short and integral with the vane drum end cover. This is secured to the vane drum with a number of countersunk screws and aligned by dowels.

Note also the vanes attached to their carriers by iron rivets. These carriers rotate on the the vane shaft which is attached to non-drive end casing, and is unsupported at the drive end of the supercharger.

THE Shorrocks relies on a close clearance between the vanes and the supercharger body (0.004") for efficiency. The vanes are a sliding fit in the trunnions which are in turn supported in the vane drum. Thus the vanes and their carriers are dragged around by the vane drum.
If the blower is run at an excessive speed, load on the drum increases dramatically and the vane shaft can flex, this allows the vanes to contact the outer casing - ouch! The results can be cataclysmic so don't go over the recommenced limits which are...... Well it depends who you are talking to, but 7000 rpm for the C75, 6000 rpm for the C142 and 5000rpm for the C250 are safe. For drag racing these may be exceeded but 7500+ is entering dangerous territory for the C75.

HERE we have the basic dimensions of the C75. Obviously they were manufactured using the Imperial measuring system, however some of the dimensions are a little odd to say the least so I've used the metric system for my convenience.

ACCORDING to all the printed information I have, threads should be BSF, but both of our blowers have UNF threads.
Inlet and outlet flanges are secured by 5/16" UNF studs. There are three 5/16" UNF threaded holes in each end cover for mounting the supercharger and these appear to be on a 5.75"PCD. The supercharger end covers are secured by a number of 1" and 2" X 1/4"UNF studs. Make sure you use a thick washer under these studs or the nut will bottom out on the thread without clamping the cover correctly.
The pulley is held on its taper by a fine thread 5/8" nut.

There are two 3/16"UNF tapped holes in the end covers so that they can be jacked off squarely with a couple of screws. Don't loose the one 3/16" dowel in each cover. The drive end vane drum cap is retained by a number of 1/4" UNF countersunk screws and located by two 1/4" dowels. Four 3/16" tapped holes are provided for jacking the cap off square.

The supercharger can be assembled with the drive at either end of the main casing. This is important as inlet and outlet ports are of different diameters and the supercharger must be configured correctly. The larger port is the inlet and rotation should be in the correct direction around the drum towards the outlet.

WITH the drive end cover removed and main casing withdrawn from the other end cover, you will be left with the innards attached to the non-drive end casing. Removal of the drive end vane drum cap exposes the vane shaft. A thin nut retains the vane carriers on the shaft and is locked with a split pin. Removal of these should allow the carriers to be slid off the shaft. However if the blower has been oversped you could be in trouble due to the carrier bearings galling the vane shaft. I'm still trying to figure out how to resolve this one.

NOW you have got this far check out the the original Shorrocks servicing instuctions from the 'superchargers' drop down menu above.